fix(view-context): a corner mark on the bubble, not a row under it

The sent-message proof was a collapsed <details> under the user bubble: a
row of height in every bubble that carried view context, for something that
is evidence about the message rather than part of it. It also rendered a
blank line, since the bubble is white-space: pre-wrap and the template left
a newline before the element.

Now a faint eye in the bubble's bottom-right corner, revealing the pairs on
hover (on focus for keyboard and touch). Deliberately not expandable, so
scrolling back through a conversation cannot grow a second layout, and the
popover opens downwards rather than over the message it belongs to.

The i18n key stays alive as the mark's aria-label; docs and changelog wording
updated to match.

CHANGELOG.md

@@ -29,7 +29,7 @@ release PR may merge — and a section is closed at the commit that bumps it.
   assistant reading through it. Like an attachment, what the eye sends goes to the AI
   provider together with your message — hover it (or tap it) to read exactly what would
   go out, click it to stop sharing; the choice is remembered on this device, and every
-  message shows a chip with what it carried. Very long highlights are trimmed, with a
+  sent message keeps a faint eye in its corner that shows, on hover, what it carried. Very long highlights are trimmed, with a
   note saying how much was left out — the assistant can still read the whole file itself.
   On by default.
 - Several conversations per source: open extra chats with `+`, and the tab bar you left